1. Field of the Invention
This invention generally relates to a driving circuit of a liquid crystal display (LCD), and more particularly to a driving circuit capable of providing a uniform common voltage distribution.
2. Description of Related Art
Recently, as the image display technology advances, a significant number of the traditional CRT monitors has been replaced by the flat panel displays. Among the flat panel displays, the thin-film transistor liquid crystal display (TFT-LCD) is most popular. In addition, the flat panel display using the light-emitting diodes or plasma has also become more popular than ever.
The display part of the flat panel display comprises a pixel array. The pixel array generally is a row-column matrix. The pixels are controlled by the drivers. The drivers drive the corresponding pixels according to rasterized image data, and the pixels will display the designated colors at prescribed time.
The LCD panel comprises an ITO layer. The ITO layer is connected to the common voltage. As the size of the TFT-LCD panel increases, the length of the layout of the common voltage becomes longer. Hence, the uniformity of the common voltage distribution on the ITO layer becomes worse. This non-uniformity of the common voltage distribution can be solved by reducing the resistance of the ITO layer. Additionally, improvement of the common voltage supply and response can further improve the flicker, and improvement of the panel and driver layout can improve the uniformity of the common voltage distribution. However the common voltage drop cannot be compensated effectively due to its inherent structure.
FIG. 1 is a traditional common voltage wire layout and the voltage adjustment circuit. Referring to FIG. 1, the common voltage Vcom is obtained by dividing the power supply VDD by the adjustable resistor 18a and amplified by the OP buffer 18b to drive the load on the entire panel 12. The fine tuning of the above voltage divider can be implemented by the mechanical trimmer of the resistor. Because the best common voltage for each panel would be slightly different, before the panel is shipped out of factory, the common voltage adjustment is necessary. The trimmer 18c is generally disposed on one end of the driving circuit board 14 as shown in FIG. 1. The OP buffer 18b and the resistor trimmer 18a/18c are disposed on the other end of the driving circuit board 14 and the panel 12. The common voltage wire 16 on the glass substrate of the TFT-LCD is coupled to the buffer 18b via the driving circuit board 14 from the source driver side.
Under this structure, the output voltage of the buffer 18b will be sent to every points (e.g., points A, B, and C) on the panel 12 via the common voltage wire 16. The fixed common voltage Vcom will drop for example from point A to point C due to the common voltage wire 16 and the panel 12 so that the common voltage distribution on the panel 12 is non-uniform.
Therefore, how to enhance the display quality and improve the uniformity of the common voltage distribution is very important. Due to the traditional circuit characteristics, the voltage drop of the common voltage Vcom cannot be improved effectively. How to modify the common voltage wire and circuit becomes an important issue.